Abstract [en]

A bistridentate RuII-polypyridine complex [Ru(bqp)2]2+ (bqp = 2,6-bis(8'-quinolinyl)pyridine) has been prepared, which has a coordination geometry much closer to a perfect octahedron than the typical Ru(terpyridine)2-type complex. Thus, the complex displays a 3.0 mus lifetime of the lowest excited metal-to-ligand charge transfer (3MLCT) state at room temperature. This is, to the best of our knowledge, the longest MLCT state lifetime reported for a RuII-polypyridyl complex at room temperature. The structure allows for the future construction of rod-like, isomer-free molecular arrays by substitution of donor and acceptor moieties on the central pyridine units. This makes it a promising photosensitizer for applications in molecular devices for artificial photosynthesis and molecular electronics.

Abstract [en]

Processes where a molecule absorbs visible light and then converts the solar energy into chemical energy are important in many biological systems, such as photosynthesis and also in many technical applications e.g. photovoltaics. This thesis describes a part of a multidisciplinary project, aiming at a functional mimic of the natural photosynthesis, with the overall goal of production of a renewable fuel from sun and water. More specific, the thesis is focused on design and photophysical characterization of new photosensitizers, i.e. light absorbers that should be capable of transferring electrons to an acceptor and be suitable building blocks for supramolecular rod-like donor-photosensitizer-acceptor arrays.

The excited state lifetime, the excited state energy and the geometry are important properties for a photosensitizer. The work presented here describes a new strategy to obtain longer excited state lifetimes of the geometrically favorable Ru(II)-bistridentate type complexes, without a concomitant substantial decrease in excited state energy. The basic idea is that a more octahedral coordination around the Ru will lead to longer excited state lifetimes. In the first generation of new photosensitizers a 50-fold increase of the excited state lifetime was observed, going from 0.25 ns for the model complex to 15 ns for the best photosensitizer. The second generation goes another step forward, to an excited state lifetime of 810 ns. Furthermore, the third generation of new photosensitizers show excited state lifetimes in the 0.45 - 5.5 microsecond region at room temperature, a significant improvement. In addition, the third generation of photosensitizers are suitable for further symmetric attachment of electron donor and acceptor motifs, and it is shown that the favorable properties are maintained upon the attachment of anchoring groups. The reactivity of the excited state towards light-induced reactions is proved and the photostability is sufficient so the new design strategy has proven successful.

Jäger, Michael

Abstract [en]

This thesis describes ruthenium(II) polypyridyl-type complexes tailored for artificial photosynthesis. Inspired by Nature, the primary events in photosystem II are mimicked by donor-photosensitizer-acceptor (D-P-A) assemblies. The photosensitizer plays a key role in such processes, and the combination of structural and photophysical properties is essential to control the electron transfer steps. In the first part, the general requirements for photosensitizers are discussed.

The second part deals with [Ru(bpy)3]2+-benzoquinone (Q) dyads (bpy is 2,2´-bipyridine) based on an asymmetric 5,5´-bisamide substituted bpy. Rapid electron-transfer from the excited state is observed to generate the RuIII-Q- charge separated states but preliminary results show no effect of the directionality of the amide link.

In the main part, a strategy to overcome the photophysical limitations of RuII bistridentate complexes (e.g. [Ru(tpy)2]2+, tpy is 2,2´:6´,2´´-terpyridine) is explored. The prototypical [Ru(dqp)2]2+ complex (dqp is 2,6-di(quinolin-8-yl)pyridine) is synthesized which displays a 3000 ns excited state lifetime at room temperature, reversible redox chemistry and high photostability. The synthesis of 4-substituted dqp is achieved via SUZUKI coupling using 8-quinoline boronic acid or ring-formation of the central pyridine. A markedly rich Ru coordination chemistry was observed, e.g. facial and meridional isomers of [Ru(dqp)2]2+. Using a chloride-free [Ru(dqp-R)(MeCN)3]2+ intermediate allows the synthesis of heteroleptic meridional [Ru(dqp-R)(dqp-R’)]2+ (R,R’ = -H, -CO2Et, -NH2, -OMe, -Br, -PhBr, …) in high yields. The meridional complexes show long-lived luminescence (450 - 5500 ns) and reversible redox chemistry. The photochemical reactivity has been investigated in typical electron-transfer reactions, e.g. in a supramolecular P-A dyad and in a multimolecular approach using biomimetic components (Mn and Fe complexes).

The dqp ligand is further used to synthesize FeII, RhIII, cyclometallating RuII complexes and an aza-analogue of [Ru(dqp)2]2+ and is discussed in the final part. These complexes were prepared with the aim to further tune the redox properties while maintaining good photophysical properties.